|- by Bruce Cairncross|
For some time now, outstanding crystals of aegirine have been collected at “Zomba”, Malawi (see Fig. 1). These specimens were reported in the early 1990s (Wilson 1991; Petersen and Grossman 1991; Robertson and King 1991). This aegirine was associated with aesthetic smoky quartz, microcline, and zircon. However, with further collecting in more recent years, several other rare species such as parisite, epididymite, fergusonite, and eudyalite, amongst others, have also been discovered. Although the occurrence of these minerals has previously been published in some detail (Petersen and Grossman 1994), much new material has since been collected and made available to the collecting market. Furthermore, the exact locality of the pegmatites was never clearly described or recorded until recently, and most, if not all, specimens have merely been labeled “Zomba, Malawi.” Zomba is a village located south of Zomba Mountain. A few South African collectors have made trips to the locality to obtain specimens at the source and to determine the exact locality of the pegmatites (Cairncross et al. 1999; Cairncross 2000).
HISTORY AND GEOLOGY
The geology of the region that contains the aegirine-bearing pegmatites has been fairly well documented. During the 1950s and early 1960s, the Geological Survey of Nyasaland (now named Malawi) embarked on extensive mapping projects in the Zomba area and several publications were written based on this work. The best reference for this period is Bloomfield (1965) who gives a detailed bibliography of all previous work up until 1965. Bloomfield’s publication deals specifically with the Zomba region and he mentions the presence of pegmatites containing large aegirine crystals. Two more readily accessible articles from this era are those of Stillman and Cox (1960) and Vail and Monkman (1960). During the mid-1980s to early 1990s more research took place, and this involved detailed petrographical and geochemical studies on the syenites, alkaline, and peralkaline rocks (Platt and Woolley 1986; Platt et al. 1987; Woolley and Platt 1986). More recently, further detailed mineralogical research has been undertaken (Woolley 1991: Woolley and Jones 1992; Jonsson and Högdahl 1999). There have also been overviews of the minerals and collectable minerals found in the Zomba area (Petersen and Grossman 1994; Cairncross et al. 1999; Cairncross 2000).
The area of mineralogical interest falls into a
part of the Cretaceous Chilwa Alkaline Province which, as its name suggests,
consists predominantly of alkaline intrusives and extrusives associated with
the East African rift (Eby et al. 1995). The Chilwa province consists of
several granite, syenite and nepheline-syenite plutons that are associated with
extrusive carbonatites and agglomerates. The Zomba-Malosa Mountains are
pear-shaped and are geologically subdivided into two zones. The southern Zomba
area consists mainly of syenite surrounded by a periphery of alkaline granite.
The northerly-located Malosa section is composed of a mixture of syenite and
granite. North and northeast of the Zomba-Malosa Mountains are four other
geological complexes named, from west to east, Chinduzi, Mongolowe, Chaone and
Chikala (see Fig. 2). These are essentially nepheline syenite complexes. From
the collector’s standpoint, two important types of pegmatites are present in
the area. In the Chinduzi-Chikala range of mountains, nepheline-syenite
pegmatites occur. These contain large well developed aegirine crystals “….. up
to 5 by 2.5 cm in size…..” as reported by Bloomfield (1965). In contrast,
granitic pegmatites are found in the Zomba Mountain and Malosa Mountain. The
northwestern fault scarp in particular has an abundance of these pegmatites and
it is from these deposits that the best specimens have been collected. To the
south, the Zomba section of the complex has far fewer pegmatites.
In order to get to the pegmatites, one has to hike about 20 km from the nearest road. This involves descending a steep slope down Zomba Mountain into a deep valley, and then hiking up the opposite side onto the Malosa plateau. The exact site where the crystals are excavated is dangerous. It is located on the northwestern side of the mountain where a series of major faults cut through the mountain. These have produced scarps with vertical cliff faces up to 800 meters high. The pegmatites that contain the minerals strike across the mountain and down the vertical cliffs so that some of these have to be reached by climbing down the near-vertical rock face. Because the quartz-bearing pegmatites tend to be more resistant to weathering, they form narrow ridges or “noses” along which one has to balance and walk.
The scarp involves a
descent down a 60º - 70º slope and the pegmatite veins average approximately 1
– 1.5 meters wide. The local miners have been digging and collecting here for
several years and have moved many tons of overburden soil and waste rock. They
do this by using the most primitive equipment such as machetes, iron rods and
hoes. Some dig only with their bare hands. There are no mines operating in the
region; all the specimens are dug from outcrop. The task of extracting minerals
therefore requires a lot of physical digging and clearing away of unwanted soil
and overburden. Red, oxidized soil overburden tumbles down the steep slope and
tails off into the valley below. These red scars mark sites where excavation
has already reached an advanced stage. Climbing equipment such as ropes and
pitons are unheard of, so the fact that many fine crystalline specimens are
collected under such dangerous conditions is quite remarkable. The aegirine-bearing
pegmatite host-rock consists mainly of quartz and potassium feldspar
(microcline). The enclosing country rock is composed of very weathered syenite.
Most of the weathered material is very soft and easy to move away. The better
quality and larger specimens are reported to come from further down the 600 –
800 meter cliff faces.
The local "miners" who dig out the specimens often set up an informal
market in the bush, miles away from any town or village. Here, minerals can be
bought. Approximately 40 – 50 miners are usually present, all with their
respective specimens, ready to trade. The local Malawians are miners in the
truest sense, because the Malawi government has granted them mining
certificates to prospect for minerals and several of them also have legal
claims to the sites where the minerals are excavated. Specimens range in size
from thumbnail, single crystals, to museum-sized matrix specimens of aegirine,
quartz and feldspar that can weigh over 15 kg.
DESCRIPTION OF MINERALS
Table 1 provides a list of the minerals that are reported from the region. Those species that are of collector interest are described below. Detailed descriptions of the other minerals can be found in Peterson and Grossman (1994) and Cairncross et al. (1999).
Aegirine, occurs abundantly in very aesthetic crystals. These vary considerably in their crystal form, size and association with other mineral species. Extremely lustrous coal-black, prismatic, single crystals up to 24 cm are known but composite crystals up to 40-50 cm have also been collected. Crystal terminations can be a simple pinacoid, but often, very steep hemipyramids impart distinctive tapering ends to the crystals, resulting in terminations in sharp points. Groups of elongate, prismatic aegirine crystals can form haystack-like aggregates. Some aegirine crystals are studded with white microcline crystals. Others are intergrown with prismatic, doubly terminated, beige zircons. Some quartz crystals have inclusions of very acicular, hair-like microscopic crystals of dark-green to black aegirine that impart black phantoms to the otherwise clear quartz. Other included quartz crystals have orange-brown hairlike inclusions. These have been analyzed and are also fibrous aegirine that has been partially oxidized.
Arfvedsonite, occurring as, sharp prismatic crystals is one of the most interesting minerals obtained. These are black and display typical aligned striations on the prism faces, parallel to the c-axis. Stubby crystals 4 –5 cm long are common but some are over 10 cm, although these were weathered and corroded. Associated species include microcline, zircon, aegirine, and quartz. Arfvedsonite is also found as fine hair-like crystals included in quartz, giving the quartz a dark-green color. Incipient alteration of arfvedsonite to riebeckite is relatively common (Petersen et al. 1994).
Caysichite-(Y), known from two specimens have been identified (Massanek 1999). These were originally thought to be natrolite but have been analyzed and shown to be caysichite -(Y). The crystals are up to 1.1 cm long, pseudotetragonal and have a milky outer layer and transparent inner core. The overall color is pale rose-pink. The mineral occurs together with smoky quartz, aegirine and orthoclase.
Epididymite, was known several years ago, from a few small (millimeter-sized) epididymite crystals, observed on some specimens and as inclusions of white, acicular crystals in smoky quartz. However, larger epididymite crystals are now also known. A terminated hexagonal crystal 4.5 cm long and 4 cm in diameter, on a matrix of aegirine and smoky quartz is in the Farquharson collection in Johannesburg. Similar large crystals are in the Messner collection in Johannesburg (now in Austria, 2004 – Editor). These crystals are colorless to white, pale-cream. Other specimens of epididymite are more typically elongate, thin (<1 mm) prismatic crystals, very commonly included within quartz crystals.
Fergusonite-(Y), is rare, and not many fergusonite-(Y) crystals are known; the species remains a rarity from the locality. Crystals usually are grouped together in bundles less than 1 cm. These display the typical tapering habit and are pale yellow. All of the fergusonite-(Y) crystals are associated with aegirine.
Galena, was until recently, never been reported from the Zomba region, but found in amongst many hundreds of aegirines was a small galena specimen. At first it was thought that the galena was collected at some other locality in Malawi and merely added into the Zomba material for sale. However, on cleaning the specimen and examining it, the presence of small, attached aegirine crystals confirmed its source as indeed originating from Zomba.
Goethite, is prevalent on most of the specimens that come from the area. It forms a brown rust-colored coating to most specimens. An interesting occurrence of goethite was found on a few specimens - goethite pseudomorphs after what is believed to be parisite. The crystals that are being replaced are flat, tabular, and hexagonal in outline, but are now composed entirely of iron hydroxides.
Another relatively new discovery is attractive, sharp cubic crystals of pyrite, up to 1 cm on edge, now altered to goethite. These occur as groups of crystals, some nucleated onto aegirine crystals.
Parisite-(Ce), has been reported from the Zomba massif. One of the largest crystals to date is a terminated, 3.2 cm X 2.1 cm hexagonal crystal associated with aegirine and a second, smaller naturally etched parisite. The parisite displays a common feature of the Zomba specimens, having a tapering, prismatic profile towards the flat, pinacoidal termination. Most of the Zomba parisite crystals are an attractive yellow color, and have distinctive, naturally etched and corroded surfaces. Small (< 2 mm), highly corroded, almost formless parisite crystals can be relatively common, scattered on a matrix of aegirine, quartz and potassium feldspar. Rarely, some crystals are transparent.
Potassium feldspar, is common at Mount Malosa. Although no definitive analyses were carried out, a few random feldspars have been examined and proved to be microcline. One of the largest crystals known measures 25 cm on edge. Groups of microcline are common and often associated with quartz, smoky quartz, aegirine and arfvedsonite. The feldspar is white to very pale-cream and Manebach twinning is very common. Some K-feldspar crystals have pitted or corroded crystal faces.
Quartz, is typical within any batch of Zomba material. The crystal sizes range from thumbnail to over 35 cm. The quartz is usually clear and colorless. However, inclusions of acicular aegirine produce almost black crystals. Smoky quartz is also common. The habits of some of the quartz specimens are somewhat unusual. Most have the typical prismatic hexagonal shape, but some crystals are extremely flattened while others display steep, pyramidal tapering towards the termination. Many quartz crystals have overgrown aegirine. A few quartz specimens have included yellow to dull-orange acicular crystals, identified quantitatively as aegirine. Quartz is associated most commonly with aegirine and potassium feldspar, as well as zircon. A few crystals of pale-purple amethyst are also known. Recently (mid-2000), interesting pseudomorphs of quartz after another hexagonal mineral have been found. Drusy quartz has replaced what is believed to be large apatite crystals, some over 15 cm long, although unreplaced apatite has not yet been reported from the deposit, so this identification remains unproven.
Scheelite, as two loose, subhedral crystals, was collected in 1999. These are an attractive orange color (similar to those from China). To date, these remain the only known scheelite specimens from Zomba.
Siderite, occurs as a few specimens of rhombohedral, tan-colored siderite crystals, forming the matrix to quartz and feldspar.
Zircon, in crystals up to 4 cm have been collected. Some are opaque and light brown, but others are gemmy, transparent orange crystals. The abundance of zircon, occurring as loose crystals as well as composite aggregates, and as zircon associated with most of the other minerals found at Zomba, attest to it being relatively common.
During late-1998 and 1999, trips were made to the Zomba massif that produced some new and interesting minerals. There are at least a dozen or more, small (1-2 mm) well-crystallized minerals that are as yet unidentified and awaiting quantitative analyses. However, what appear to be small 2-3 mm cubic crystals of pale-blue fluorite have been positively identified. There are no doubt many interesting micromount minerals that have been overlooked in the past because the local Malawi digger’s tend to only concentrate on the larger, more commercial quartz and aegirine specimens and are not even aware of the rare micromount potential of the minerals of Zomba.
Access to the area from where the collectable minerals are found is not easy. One has to carry in all provisions, camping, and digging equipment because no facilities or shops exist on the mountains. In order to spend a few days collecting, one has to camp on the summit of the mountain – it is not possible to hike in and out every day. The mineral specimens are all collected from outcropping pegmatite veins. Collecting the crystals is hazardous in the extreme. Most of the more easily accessible pegmatites have been excavated and depleted of minerals. The remaining pegmatites are those that crop out on the very steep scarp faces of the northwestern part of Malosa Mountain. Local Malawians climb these treacherous slopes barefoot, hanging precipitously above drops of several hundred meters to the valley below. Recently (late-2000), the Malawi Police have begun patrolling the area, asking any foreigner for the compulsory permits and documentation that are required by Malawi law to collect and deal in mineral specimens. If one does not possess these documents, the resulting penalties can be severe, even resulting in imprisonment.
Table 1: List of minerals reported from the Zomba-Malosa region.
|Bastnäsite - (Ce)||(Ce,La)(CO3)F|
|Caysichite - (Y)*||Y4(Ca3REE)(OH)(H2O)5(Si8O20)(CO3)6•2H2O|
|Fergusonite - (Y)*||YNbO4|
|Hingganite - (Y)||(Y,Yb,Er)BeSiO4(OH)|
* = Species discussed in the text.
(List compiled from Bloomfield 1965; Petersen and Grossman 1994; Petersen et al. 1994; Cairncross et al. 1999 and Johnsen et al. 1999 and personal observations).
Note: rutile has been deleted from the list on Zomba minerals. The original, visual identification of this species was made by Petersen and Grossman (1994), who reported on reddish-brown hairlike crystals included in quartz. Analyses have since shown these inclusions to be aegirine.
Prof. Bruce Cairncross, Department of Geology, Rand Afrikaans University,
P.O. Box 524, Auckland Park, 2006, South Africa.
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Cairncross, B., Messner, K. and Farquharson, E. 1999. Die pegmatite des Mount Malosa bei Zomba, Malawi. Lapis 24(4): 22-32.
Cairncross, B. 2000. Minéralogie des pegmatites du mont Malosa, district de Zomba, Malawi. Le Règne Minéral 35: 27-37.
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Johnsen, O., Ståhl, K., Petersen, O. V. and Micheelsen, H. I. 1999. Structure refinement of natural non-metamict polycrase-(Y) from Zomba-Malosa complex, Malawi. Neues Jahrbuch für Mineralogie Monatsheft 1: 1-10.
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Massanek, A. 1999. Neu aus Malawi: Caysichit-(Y). Lapis 24(4): 33.
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